1. Field of the Invention
The present invention relates to a humidifier, and more particularly, to an evaporative humidifier for a fuel cell system capable of humidifying gas supplied to a fuel cell system.
2. Description of the Background Art
A fuel cell is being spotlighted as a non-polluting alternative energy system. A Polymer Electrolyte Membrane Fuel Cell (PEMFC) among the fuel cells is operated at a low temperature less than 80° C., and is able to start up in a short time. Furthermore, the PEMFC has a fast response characteristic according to a load variation. Accordingly, the PEMFC is being widely applied to various fields including an automobile, a mobile communication device, a medical equipment, etc.
The PEMFC generates electricity accordingly as oxygen and hydrogen are respectively supplied to a cathode and an anode of a polymer membrane inside a fuel cell stack. That is, hydrogen, fuel gas is supplied to the anode of the fuel cell stack, and oxygen is supplied to the cathode of the fuel cell stack. A polymer membrane for passing only proton is disposed between the anode and the cathode. The proton moves from the anode to the cathode through the polymer membrane, and electron (e−) is separated therefrom, thereby generating electricity. The oxygen supplied to the cathode reacts with the proton having passed through the polymer membrane, thereby generating water and heat.
In order to efficiently operate the fuel cell, the polymer membrane has to contain a certain amount of moisture therein. When an amount of moisture is less, proton conductivity is degraded and the membrane is contracted. On the contrary, when the amount of moisture is excessive, flooding occurs thus to lower an electrochemical reaction rate. In order to control the moisture content of the polymer membrane properly, a humidifier is required for humidifying gas supplied to the fuel cell stack.
A humidifying method of the humidifier includes ultrasonic vibration method using ultrasonic vibration, a bubbling method using a vaporization by external heating, a humidifying method using a membrane, etc. The conventional humidifying methods have the following problems.
First, in the ultrasonic vibration method, water emitted by ultrasonic vibration is not small enough to perform an efficient humidifying operation. Furthermore, since liquid droplets are attached to an inner surface of a humidifier, it is difficult to constantly and precisely perform a humidifying operation.
Second, in the bubbling method, an additional heating system for vaporization has to be provided, and a heating energy has to be supplied. Also, in case of a spraying method, an additional system and energy for compressing water to a high pressure are required.
Third, in the humidifying method using a membrane, moisture of exhaust gas is transferred to supply gas through a membrane that transmits only the vapor but not the gas. Even if additional energy is not required in the method, a surface area of a membrane has to be sufficiently large for an efficient humidifying operation. In this respect, a bundle of fine tubes with a diameter of approximately 1 mm made of the moisture exchange membrane is generally used in a similar way to a shell-and-tube heat exchanger. However, since the membrane is expensive and is not easily processed, this type of humidifier is not suitable for mass production thus to increase the production cost of a humidifier. Moreover, since the humidifier in this method is very complicated in structure, the pressure loss tends to increase enormously especially in the gas stream flowing inside the tubes Recently, a humidifying method requiring no additional energy and utilizing evaporation from a hydrophilic-treated surface has been disclosed in the U.S. Pat. Nos. 6,779,351, and 6,871,844.
However, according to a general humidifying method, a condenser to recover water from the exhaust gas for humidification has to be additionally provided thus to increase an entire size of a humidifying system.
Second, according to the humidifying method disclosed in the U.S. Pat. No. 6,779,351, since extended surface is not implemented, channels have to be arranged with a narrow gap therebetween for efficient heat transfer. Herein, the gas to be humidified is introduced into an evaporation channel through small holes, thereby greatly increasing pressure loss of the gas stream. Furthermore, since water does not efficiently spread on the surface of the evaporation channel, it is difficult to obtain sufficient humidification effect with this method.
Third, according to the humidifying method disclosed in the U.S. Pat. No. 6,871,844, the pressure loss tends to increase in the gas stream in the evaporation channel due to a moisture-absorbing material irregularly packed in the channel. Furthermore, since the moisture tends to condense mostly on a surface of a fin, most of the condensed water flows down along the surface of the fin and only small portion of the water transfers to the evaporation channel through a hole arranged at the channel wall. Accordingly, the condensed water tends to accumulate at a lower side of the condensation channel without being supplied to an evaporation channel. Moreover, when the pressure in the evaporation channel is higher than that in the condensation channel, the condensed water does not transfer from the condensation channel to the evaporation channel but, rather, the gas leaks from the evaporation channel to the condensation channel due to the pressure difference between the channels.